1. Field of the Invention
The present invention relates a method of forming a coating film such as a resist film on a semiconductor wafer or a layer formed thereon, and an apparatus therefor.
2. Description of the Related Art
As is well known, in the field of semiconductor techniques, a resist film is formed on the surface of a layer as a mask for a pattern portion when a semiconductor layer, an insulating layer, or an electrode layer formed on a semiconductor wafer is to be etched into a predetermined pattern.
For example, spin coating is known as a method of forming a resist film. According to this method, a table is rotated with a semiconductor wafer being placed and fixed thereon. For example, a resist solution consisting of a solvent and a photo-sensitive resin is dropped on the central portion of the upper surface of the wafer. The resist solution is spirally diffused from the central position of the wafer to its peripheral portion in accordance with the rotational and centrifugal forces of the wafer, thereby coating the resist solution on the semiconductor wafer.
According to this method, in the process of diffusing the resist solution from the central position of the wafer to its peripheral portion, the solvent in the resist solution evaporates. For this reason, the viscosity of the resist solution varies in a diffusion direction, and the thickness of the resist film at the central portion is different from that at the peripheral portion. In addition, the peripheral velocity at the peripheral portion is higher than that at the central position, and a scattering amount of the resist solution at the peripheral portion is much larger than that at the central position. Thus, uniform coating can not be obtained.
For this reason, as described in Jpn. Pat. Appln. KOKAI Publication Nos. 57-43422 and 59-141220, there is provided a method of adjusting the temperature of a resist solution or a method of filling the same solvent as in a resist solution in a resist film formation atmosphere to suppress evaporation of the solvent in the resist solution. As described in Jpn. Pat. Appln. KOKAI Publication Nos. 59-11895, 61-91655, and 61-150332, a method of dropping the solvent of a resist solution on a wafer surface prior to resist solution coating is also proposed.
In the former method, however, the amount of resist solution used in forming a resist film is large. For example, only 1 to 2% of the total amount of resist solution contribute to actual formation of the resist film. For example, to form a 1-.mu.m thick resist film, 4 to 8 cc of a resist solution are required for an 8" semiconductor wafer. In the latter method, it is difficult to obtain uniform coating and thus, the conventional problems cannot be satisfactorily solved.